The general principle of an angle detecting device, such as a rotary encoder, is as follows. The angle detecting device is a device having scale marks written at a circumference of a circular scale disc, and a sensor head for counting a scale, to count the number of scale marks, thereby outputting angle information. Various kinds of devices are used as the angle detecting device. Since the angle detecting device has scale marks written artificially, the scale lines are not written equiangularly, thus the angle information obtained from the positions of the scale lines involves an error. In FIG. 1, a radial line L1 is an ideal scale line position (equiangular interval line), and a short radial broken line L2 is an actual scale line position. A graph in the right of FIG. 1 is obtained by plotting a difference from the ideal position.
The points in the drawing in the right in FIG. 1 are calibration values of scale lines of the angle detecting device. FIG. 1 shows 36 scale lines as depicted, but actually, the angle detecting device has several thousand to several hundred thousand scale lines. A method for calibrating these lines includes several methods for self-calibrating the lines, by comparing the scales of the two angle detecting device with each other. In this method, even if the two angle detecting devices are not calibrated, they can be calibrated at a time, and thus it is not necessary to prepare a more accurate angle detecting device at a higher level. It should be noted that the meaning of the self-calibration is that, even when two angle detecting devices of which angular errors are unknown are compared, the calibration values which are angular errors of both of the devices can be found at the same time.
In the national standard device of angle (angle measuring device), an angle detecting device residing inside of the angle measuring device and an angle detecting device to be calibrated that is provided thereabove, are calibrated using self-calibration method according to the equal-division-averaging method.
The equal-division-averaging method will be simplified and briefly explained with reference to FIG. 2. A difference (SA1) of a scale signal is measured between one of first sensor heads 12, 12 . . . arranged on a scale disc of a first angle detecting device 11 at a lower side and a second sensor head 14 arranged on a scale disc of a second angle detecting device 13 at an upper side. Then, a difference (SA2) is measured in the same manner between another adjacent sensor head 12 at the lower side and the second sensor head 14 at the upper side. Likewise, differences (SA1, SA2, SA3, SA4, SA5) are measured between other first sensor heads 12 and the second sensor head 14, and an average value SAV of those differences is determined, so that a calibration curve of the second angle detecting device 13 at the upper side can be obtained.
When an angular error that is output from the first angle detecting device 11 at the lower side is denoted as ai, and an angular error that is output from the second angle detecting device 13 at the upper side is denoted as bi, the difference is SAj=bi−ai+(j-1)N/M and, and the average value SAV is as follows.
                              SAV          i                =                ⁢                              1            N                    ⁢                                    ∑                              j                =                1                            N                        ⁢                          SAj              i                                                              =                ⁢                              b            i                    -                                    1              N                        ⁢                          (                                                a                  i                                +                                  a                                      i                    +                                          P                      /                      N                                                                      +                ⋯                +                                  a                                      i                    +                                                                  (                                                  j                          -                          1                                                )                                            ⁢                                              P                        /                        N                                                                                            +                ⋯                +                                  a                                      i                    +                                                                  (                                                  N                          -                          1                                                )                                            ⁢                                              P                        /                        N                                                                                                        )                                          Herein, i=1, 2, 3 . . . , N is the number of the scale line, N denotes the total number of scale marks provided on the scale disc.
When five first sensor heads 12 are provided, the first sensor heads 12 are arranged with an angular interval of one-fifth of 360 degrees around a circle. When N angle detecting devices are provided, the first sensor heads 12 are arranged with an equal angular interval of one N-th in the same manner. This is called the equal-division-averaging method.
The applicant (assignee) proposed an angle detecting device with a self-calibration function, in which, as shown in Patent Literature 1 below, when calibrating the angle detecting device, the angle detecting device can constantly make accurate calibration, by making it possible to obtain calibration values of a scale including, e.g. the influence of axis eccentricity of the angle detecting device itself and change of the angle detecting device with the aging variation, in order to eliminate the error caused by the axis eccentricity occurred in the time of connection of a rotating shaft with the angle detecting device in a calibration device, and in which the size of the angle detecting device can be reduced.
According to one described in Patent Literature 1, an angle detecting device having sensor heads at a circumference of a scale disc fixed to a rotating shaft, includes a plurality of first sensor heads at the circumference of the same scale disc, and a second sensor head, in which measurement differences between the second sensor head and the first sensor heads are obtained to determine an average value thereof, to perform self-calibration on the basis of this average value, whereby the angle detecting device can detect, e.g. the calibration values of the scale as well as errors due to connection of the rotating shaft, and moreover, even while the angle detecting device is used, calibration operation can be performed readily as necessary without performing connection operation of the rotating shaft, and accurate measurement can be performed for a long period of time in a stable manner.